Establishment Republicans in three neighboring East Coast states are hoping to throw a monkey wrench into Donald Trump’s breakaway presidential campaign. Though he has been the party’s front-runner since shortly after announcing his candidacy, Republicans in Virginia, North Carolina and South Carolina are considering a requirement for primary candidates that he might not be able to meet.

In the first few minutes of the first GOP presidential debate, Trump was the only candidate on the stage to acknowledge that he could not rule out a possible third-party candidacy should he not win the Republican nomination.

Many in the party believe such a bid would siphon votes from Republicans and ensure a Democrat victory. Party leaders in the aforementioned states, therefore, are considering forcing any Republican candidate, prior to appearing on state primary ballots, to vow he or she will not pursue a third-party candidacy.

South Carolina was the latest to join the coalition. In that state, candidates must affirm their belief in and support of “the nominees and platforms of the Republican Party in the November 8, 2016 general election” prior to being included in the primary election. Seven candidates have already signed or are prepared to sign the pledge within the next few days.

Trump, however, has yet to back away from his threat to run an independent campaign.

While those behind these proposed rules are ostensibly trying to protect the party, at least one former Trump adviser contended that the plan could have the opposite effect.